When I yaw my gimbal with the RC the roll axis is being effeckted and starts rolling. It slowly comes back to zero again. I have tried to cut the power to the roll motor and can then see in the software that the roll axis i changing even that the roll axis does not move.When I power on the roll motor its rolling the gimbal when I yaw. WHY

How is your IMU installed, it needs to be in level when pitch and roll are in level (or exactly 90 degrees to level) How is your IMU calibrated? Do you have Follow mode activated? how is your gyro trust?

If you want to keep a certain axis fixed try setting 'I' to 0 and keeping power on.